The objective of this study is to assess the interaction of three intrinsic variables - pre-load, after-load, and heart rate, each of which independently influences cardiac performance. To date the interaction of these three variables has received little attention. Increases in performance produced by serial increments of one these determinants will be evaluated against a background of several levels of elevation of one of the other variables. The hemodynamic response will be examined: 1) to determine whether or not the background elevation of one variable produces larger increments of performance for given increases of another variable, 2) as to the relative positions of the peaks of performance, and 3) as to the relative positions of the descending limb of the curve relating performance to increases of one of the variables. Experiments will be carried out under controlled conditions using both ejecting heart and isolated cardiac muscle preparations. The information gained will further the physiologist's understanding of cardiac function; it will assist the clinician in his evaluation of patients with an underlying alteration of either heart rate, end-diastolic pressure, or aortic pressure. The principles involved should aid the bio-engineer in the development of an artificial heart.